The carbon cycle, also known as the carbon-nitrogen-oxygen cycle, is one of two main series of nuclear reactions through which main-sequence stars release nuclear energy in their cores by converting hydrogen into helium. The proton-proton reaction is dominant for low-mass main sequence stars, including the Sun. The carbon cycle is the primary energy source for main sequence stars of large mass. It consists of the following reactions, where the superscripts represent the number of protons in an atom of carbon (C), hydrogen (H), nitrogen (N), oxygen (O), or helium (He):
Neither carbon nor nitrogen is used up by this set of reactions, but the presence of at least one is necessary. Four of the nuclear reactions involve a carbon or a nitrogen nucleus combining with a proton (xH) to form a new nucleus. The other two reactions consist of the decay of radioactive nuclei. These radioactive decays release both positrons and neutrinos. Positrons are "antielectrons"; they quickly find electrons and mutually annihilate each other with the release of gamma rays. Neutrinos have little interaction with other particles; they escape from the star with part of the energy of the nuclear reactions. The net effect of the carbon cycle is to convert four protons and two electrons into a helium nucleus, plus two neutrinos plus gamma radiation.
The radiation released through the carbon cycle replaces the energy radiated into space. This replacement allows a star to maintain a balance of energy as long as its fuel supply lasts. When the hydrogen at its core is exhausted, the carbon-cycle reactions must stop, and the star is no longer on the main sequence. The cycle may also operate in later stages of a star's life, when hydrogen-burning reactions occur in shells around the exhausted core.